1. Field of the Invention
The present invention relates to analog to digital converters. More specifically, the present invention is directed to an integrating analog to digital converter which converts an analog signal to a corresponding digital count.
2. Description of the Prior Art
A common method of analog to digital conversion is to employ circuit techniques which translate a voltage input signal to be converted into a time duration signal which is proportional to the voltage input signal. The time duration signal is usually obtained by integrating the input signal and, subsequently, converting the triangular ramp output of the integrator into a variable duty cycle square-wave signal by a comparator operating between threshold limits. The time duration of the square-wave signal is then converted into a digital output form by counting clock cycles from a fixed frequency oscillator during the period or time duration of the square-wave signal, e.g., the analog to digital converter shown in U.S. Pat. No. 3,745,556. However, such prior art converters fail to provide a synchronization of the clock to the time duration signal to prevent false clocking of the duty cycle measuring counter. Such a lack of synchronization is effective to provide a false count by .+-.N clock pulses where N is the number of triangle wave cycles at the output of the integrator which results in an error in the digital representation of the input signals to be converted. Accordingly, it is desirable to provide an analog to digital converter having means for overcoming the aforesaid inherent disadvantage of the prior art converters.